The concept of integrating a solar thermal power plant into a fossil fuel power plant has previously been investigated; in these investigations, solar heat was used to preheat feedwater through evaporation of the feedwater or the feedwater is bifurcated resulting in a portion of the feedwater stream to be heated with extraction steam and the remaining feedwater stream heated by solar means. For example, Cohn U.S. Pat. No. 5,727,379 teaches that solar heat is used to evaporate the feedwater prior to boiler entrance. This method would then require an expensive solar boiler and is not applicable to a Rankine cycle using regenerative heating. Warren U.S. Pat. No. 4,069,674 teaches that the feedwater is bifurcated immediately at the condenser hotwell resulting in high temperature solar being used to heat relatively cold feedwater resulting in high entropic losses. Thermodynamically, it is always preferable to add heat at the highest possible temperature. In addition, current technology being explored uses solar heat to evaporate water into steam which is then used to directly provide energy to a feedwater heater. This type of technology results in complex and costly equipment when compared to the novelty proposed.
While all of these methods provide some sort of value addition, they all have certain thermodynamic restrictions and cost disadvantages. The following concept proposes a novelty that is more cost effective and energy efficient than any of the previously studied integration methods and poses less capital and less operational risk to the host regenerative steam cycle power plant. The concept can be used for new installations or can be easily retrofitted into existing regenerative Rankine power cycles such as coal plants. In particular, the southwest United States has numerous coal plants that are located in isolated areas that have high solar insolation that are prime candidates for solar thermal retrofit. This method may also be used for other fossil fueled (including natural gas) or nuclear fueled boiler scenarios and most other regenerative steam Rankine power cycles.
For purposes of illustration, this novelty is discussed herein as applied to a coal plant. Nearly all large coal plants use both reheat and regeneration to achieve high cycle efficiency. Reheat can be defined as returning steam, which has been partially expanded in the turbine, back to the boiler for additional heating prior to continued expansion in the turbine. Regeneration is a method to limit condenser loss in a Rankine cycle by taking partially expanded steam (extracted from the steam turbine) and using it to pre-heat the feedwater prior to additional heating and vaporization in the boiler. By pre-heating the feedwater, less heat energy is needed in the boiler to produce steam and, since the partially expanded steam is condensed using feedwater as the “heat sink”, less heat is rejected to the condenser. Regeneration can be accomplished using either “open” or “closed” feedwater heaters. In the “open” feedwater heater, the extracted steam from the turbine is mixed directly with the feedwater; in the “closed” feedwater heater, the extraction steam is not mixed with the feedwater but uses both sensible and latent heat in normally a tube and shell feedwater heater to boost the feedwater temperature.
This novelty achieves lower solar power electricity costs by utilizing the existing power plant components and infrastructure, primarily the turbine and generator of the Rankine cycle power plant. A solar boiler is also not necessary since all solar heat is directly transferred to the Rankine cycle via a common fluid that is used for both solar heat collection and heating the feedwater stream in the Rankine cycle.